An attachment of the guide tubes to the lower end fittings is illustrated in U.S. Pat. No. 4,738,820.
The guide tubes have plugs in their lower ends to facilitate attachment to the lower end fittings. The guide tubes and plugs are of zircaloy, an alloy of zirconium. The lower end fittings and cups attached thereto to receive the plugged lower ends of the guide tubes are of stainless steel, such as that known as 304. The cup acts as a mounting sleeve for the lowest grid which is also of a stainless steel, such as Inconel 625 or 718. The other grids are of zircaloy.
Since the coefficient of expansion of zircaloy and stainless steels such as 304 and Inconel 625 or 718 are significantly different, attachments between zircaloy and stainless steel elements of a structure must accommodate the tendency of the elements to move at relatively different times, rates and magnitudes. Accordingly, attachments of these elements for structures which will maintain their integrity over time are difficult. For example, fuel assemblies may be used in a nuclear reactor, typically, for as many as three cycles of eighteen (18) months each. Many temperature cycles which potentially weaken bottom nozzle to guide tube connections occur during this period of time.